1. Field of the Invention
The present invention relates to an oscillation device detecting temperature under which a quartz-crystal resonator is placed and performing temperature compensation of an output frequency based on a result of the temperature detection.
2. Description of the Related Art
TCXO is called a temperature-compensated quartz-crystal oscillator, and by combining a temperature characteristic that a quartz-crystal resonator has and an opposite circuit characteristic, it can have a better temperature characteristic over a wide temperature range than that obtained with the characteristic of only the quartz-crystal resonator. Further, not only having such a good temperature characteristic but also having advantages of its compact size, light weight, short activation time, and the like, the TCXO is used as reference oscillators in various kinds of communication devices and measuring devices such as a frequency counter and a synthesizer.
TCXO has a temperature compensation circuit including a temperature sensitive element, and as the temperature compensation circuit, available are one using a thermistor as the temperature sensitive element and having the combination of a resistor and a capacitor, and one using an element in an LSI as the temperature sensitive element. These TCXOs have merits and demerits in view of temperature compensation accuracy, phase noise, and easiness of downsizing, depending on such a difference in the structure of the temperature compensation circuit.
FIG. 13 shows an ordinary structure of the TCXO. 90 denotes a quartz-crystal resonator and 91 denotes an oscillator circuit, and by changing a control voltage supplied from a control voltage generating part 93 to a voltage variable capacitance element 92, a capacitance of the voltage variable capacitance element 92 is controlled, so that an oscillation frequency (output frequency) is adjusted.
Since the frequency of the quartz-crystal resonator 90 changes according to temperature, the control voltage generating part 93 corrects the control voltage according to a temperature detected by a temperature detector 94. Concretely, for example, a cubic function which is a function being a frequency-temperature characteristic of the quartz-crystal resonator 90 normalized on a reference temperature, is stored in a memory 95, and based on this function (frequency-temperature characteristic), a frequency corresponding to a temperature detection value is read. That is, to what degree the frequency at the detected temperature deviates from the frequency at the reference temperature is read, and a control voltage corresponding to this frequency deviation amount is subtracted as a temperature compensation amount from a control voltage corresponding to the frequency at the reference temperature. As structures of such TCXO, there have been known an H-type structure in which a quartz-crystal resonator and an IC chip including a temperature detecting element are provided on both surfaces of a ceramic substrate respectively, a single seal structure in which a quartz-crystal resonator and an IC chip including a temperature detecting element are provided in a casing, and so on.
However, in the above-described TCXO as well, frequency stability sometimes deteriorates due to a temperature characteristic of the quartz-crystal resonator 90 and a temperature characteristic of a quartz-crystal oscillator circuit composed of the quartz-crystal resonator and the oscillator circuit 91, which has created a demand for an oscillation device higher in frequency accuracy. Further, a thermistor is generally used as the temperature detector 94, and due to its limitation in temperature detection accuracy, improvement in frequency accuracy cannot be expected. Further, since the temperature detector 94 and the quartz-crystal resonator 90 are disposed at different positions, it is not possible to accurately obtain actual temperature information of the quartz-crystal resonator 90, and because of this, improvement in frequency accuracy cannot be expected, either.
Further, the aforesaid temperature compensation amounts at respective temperatures are set for each TCXO. As a method of setting this temperature compensation amount, a thermostatic oven is prepared in a production line, and when TCXO being a target product is loaded therein, its frequency is measured while temperature in the oven is varied, and the temperature compensation amounts are individually set. Before this frequency measurement, it is necessary to wait until the temperature of the TCXO becomes constant after the temperature in the oven is varied. Therefore, it takes a long time to manufacture the TCXO.
Patent Document 1 describes, in FIG. 2 and FIG. 3, that two pairs of electrodes are provided on a common quartz-crystal piece to form two quartz-crystal oscillators (quartz-crystal resonators). Further, it describes, in paragraph 0018, that a frequency difference occurs between the two quartz-crystal resonators according to a temperature change and therefore measuring the frequency difference is equivalent to measuring the temperature. Further, Patent Document 1 describes, in paragraph 0054, paragraph 0055, and FIG. 6, that a fundamental frequency f1 and an Mth overtone frequency fm which are taken out from one quartz-crystal resonator do not necessarily satisfy f1=fm/M due to a difference in vibration mode, and a frequency difference Δf having temperature dependency occurs between f1 and fm/M. Then, a relation between the frequency difference Δf and a frequency correction amount is stored in a ROM and the frequency correction amount is read based on Δf.
However, as described in paragraph 0019, this method requires the adjustment of the quartz-crystal resonators so that a desired output frequency f0 and frequencies f1, f2 of the two quartz-crystal resonators have a relation of f0≈f1≈f2, and thus has a problem that a manufacturing process of the quartz-crystal resonators becomes complicated and a high yield cannot be obtained. Further, since clocks being frequency signals from the respective quartz-crystal resonators are counted for a predetermined time and a difference therebetween (f1−f2) is found as shown in FIG. 4, the detection time has a direct influence on detection accuracy, which makes highly accurate temperature compensation difficult. Further, since the fundamental and the overtone are taken out from the single quartz-crystal resonator, the circuit becomes complicated and there is a concern about interference between an Mth spurious component of the fundamental and fm.
[Patent Document 1] Japanese Patent Application Laid-open No. 2001-292030